Waterproofing Cinder Blocks

Cinder block wall sealerCMUs are commonly used for the construction of commercial buildings, load-bearing foundation walls, and exterior retaining walls. What is a CMU? By definition, CMU is a "concrete masonry unit." Concrete blocks and cinder blocks are CMUs, as well as splitface blocks and light weight blocks.

Are concrete blocks the same as cinder blocks? There is a distinct difference in composition between the two. Standard concrete blocks are made from portland cement, sand, and gravel. Cinder blocks contain less portland cement which is partially replaced by fly ash from power stations or cement kilns. The fly ash actually adds strength but some cinder blocks release mercury and other heavy metal contaminants – another reason for sealing them if possible.

Cinder block walls can be made stronger than a poured concrete wall by using reinforcing steel and filling their hollow cores with concrete. However, they are usually more porous and more susceptible to water seepage than standard concrete blocks.

Why Cinder Block Walls Leak

Cinder blocks are more porous than standard concrete blocks or poured concrete. Only a thin wall (1-1/4 inch) separates the hollow core from wind-driven rain or wet ground, as well as the water-filled core from the indoors. If the cores are not properly filled with a slurry cement mix, cinder blocks are considered permeable to water and vapor (>10 perms). Standard concrete blocks are semi-permeable – about 5 perms or, when filled, about 2 perms.

In comparison, a good quality newly poured concrete wall 8-10 inches thick is impermeable (0.4-0.3 perms). If your foundation walls are cinder block, they are much more likely to leak than poured concrete walls once the exterior waterproofing coating deteriorates.

After the exterior waterproofing coating on basement walls deteriorates, which may be in 5 or 10 years, the blocks are on their own to fight against water pressure. A losing battle. They are permeable to water.

If you do not seal the cinder blocks now, they can become unsealable after extensive water seepage – after the seeping water has carried clay or soil through the concrete and filled the pores. The telltale sign of this would be brown or "reddish" spots on the interior surface of the wall. Water flows easily through soil but it prevents a penetrating concrete sealer like RadonSeal from penetrating and reacting.

Exterior above-grade cinder block walls are not subject to such a water pressure but wind-driven rain pushes water right into the blocks and eventually, through the wall.

How To Waterproof Your Cinder Block FoundationRadonSeal Concrete Sealer

To waterproof, strengthen, and preserve the overall appearance of your cinder blocks, seal the blocks permanently against moisture and efflorescence with RadonSeal Plus Penetrating Concrete Sealer, the strongest concrete sealer available. RadonSeal® is easy to apply by homeowners and strong enough to be guaranteed by contractors. Follow these cinder block sealing steps:

1. Remove Paint and Efflorescence – The surface of the blocks must be bare and porous. Remove any paint either mechanically (wire brushing, grinding, sand-blasting) or with our industrial strength Lightning Strip paint remover. Brush off crumbling concrete, dust, and dirt. Light efflorescence can be removed with a wire-brush or wire-wheel attachment. Alternatively you can use our safe and easy spray-on Efflorescence Cleaner.

2. Allow the Cinder Block Walls To Dry Out – If the wall appears to be damp (wet spots) allow them to dry out. Ventilate your basement or garage by opening windows and doors. Install a fan to improve airflow. If the blocks are continuously wet, drill weeping holes using a masonry bit at the bottom of each core to drain them.

3. Apply RadonSeal® Plus – Apply four back-to-back applications of RadonSeal using a common hand-pump "garden" sprayer.

4. Optional: Ion-Bond Armor – Let RadonSeal cure for at least 10 days, remove any purged efflorescence, and spray on Ion-Bond Armor Subsurface Elastomeric Concrete Sealer. This is also a penetrating concrete sealer that disappears into the concrete, where it bonds and reacts to make the waterproofing seal even tighter. RadonSeal alone is likely to do the waterproofing job on cinder blocks in good conditions but for more porous blocks or if you want to make sure, use also Ion-Bond.

Hard-to-Seal CMUs - Go The Distance

Ion-Bond Concrete SealerUsing RadonSeal with Ion-Bond to seal cinder blocks has proved foolproof for many a homeowner. However, very old or extensively leached out cinder blocks may require special steps to properly fill and seal the enlarged pores.

There are many different types of concrete blocks that vary widely among regions and manufacturers. Since RadonSeal seals concrete by reacting with cement, it cannot seal blocks with little or no portland cement. And no sealers will seal blocks with unusually large pores. Examples:

  • Lightweight Blocks – Split face blocks, Haydite blocks, and dry-pressed blocks (all being much lighter than the 38 lbs. of standard concrete blocks) They can be sealed and waterproofed with our LastiSeal Brick & Masonry Sealer.
  • "Popcorn Blocks" – made with a very course aggregate with large toothpick-size holes.

Using thinset mortar has proven successful on many apparently hopeless situations:

1. Purchase thinset mortar – Good-quality thinset mortar (without vinyl additives) is available in the tile section of home improvement centers. We recommend Mapei "Keraset Professional Grade Dry-Set Mortar", which is inexpensive (about $20 for a 50-lb. bag), spreads easily, adheres very well, and does not shrink.

2. Wet the surface and apply the mortar on the blocks –  When still wet after applying RadonSeal Plus, use the smooth edge of a 3/16" notched trowel holding it at a 45° angle to push the mortar very hard into the surface. The purpose is to push the thinset into the pores rather than building up a layer. There will be lots of droppings on the ground. You can finish it off neatly by going over the mortar joints with a round tucking tool and make it look like a brand new block wall. Or, if you prefer the look of a poured concrete wall, leave a smooth layer (say 1/8") on the surface.

3. Seal the thinset using Ion-Bond Armor– Thinset is also porous and would soon leak. RadonSeal cannot react with thinset because of additives. Allow four days for the thinset to cure and then, seal the wall with our Ion-Bond Armor in two successive applications about 20 minutes apart. Ion-Bond penetrates through the thin-set, bonds with RadonSeal inside the blocks, and permanently seals both the cinder blocks and the mortar.

A stronger alternative to the thinset method is troweling on a mix our ElastiPoxy Crack Filler with sand, particularly if the blocks are in a very bad shape or already crumbling. First, use RadonSeal Concrete Sealer to strengthen and waterproof the inside of the concrete. Use the same epoxy-sand mix to also fill cracks, holes, and other defects. The epoxy-sand mix delivers a repair that is stronger than the concrete itself. It will last forever! Check out the Picture Guide on Repairing Cracks in Concrete Block Walls.

You can also read the article "Waterproofing Concrete Block Foundations."